Improve visibility of the link buttons

[MartinBenninger]

Based on feedback from multiple users, parts of the UI aren't self-explanatory for new users.

They click on an item to view the details and then click save without first clicking the link button. The widget doesn't do anything in this case (since the link button hasn't been pressed) so it appears to not work.

This pull request introduces multiple improvements to fix this problem.

• First of all, the save buttons are disabled so long as there is nothing selected to save. This already will fix the main issue.
• Additionally, an animation has been added to the button when hovering or selecting an item. This should help to make it more visible.
• Last but not least, a message is shown when the user hasn't selected anything to link. The message has an arrow pointing towards the button that they need to press.

Fixes #32

[MartinBenninger]

This pull request was linked by RTC Work Item 695009 on behalf of martin.benninger@siemens.com